Nanoparticles having metal non-oxide compositions (i.e., “semiconductor” or “quantum dot” nanoparticles) are increasingly being used in numerous emerging applications. Some of these applications include electronics (e.g., transistors and diode lasers), LED displays, photovoltaics (e.g., solar cells), and medical imaging. Quantum dot nanoparticles are also being investigated as powerful new computer processing elements (i.e., qubits). Semiconductor nanoparticles often possess a metal chalcogenide composition, such as CdS, CdSe, ZnS, and ZnSe.
As a consequence of its small size, the electron band structure of a quantum dot differs significantly from that of the bulk material. In particular, significantly more of the atoms in the quantum dot are on or near the surface, in contrast to the bulk material in which most of the atoms are far enough removed from the surface so that a normal band structure predominates. Thus, the electronic and optical properties of a quantum dot are related to its size. In particular, photoluminescence is size dependent.
Several physical methods are known for synthesizing semiconductor nanoparticles. Some of the physical techniques include advanced epitaxial, ion implantation, and lithographic techniques. The physical techniques are generally useful for producing minute amounts of semiconductor nanoparticles with well-defined (i.e., tailor-made, and typically, uniform) morphological, electronic, magnetic, or photonic characteristics. The physical techniques are typically not useful for synthesizing semiconductor nanoparticles in commercially significant quantities (e.g., grams or kilograms).
Several chemical processes are also known for the production of metal chalcogenide nanoparticles. Some of these methods include arrested precipitation in solution, synthesis in structured media, high temperature pyrolysis, and sonochemical methods. Although the foregoing chemical processes are generally capable of producing semiconductor nanoparticles in more significant quantities, the processes are generally energy intensive (e.g., by generally requiring heating and a post-annealing step), and hence, costly. Accordingly, commercially significant amounts of the resulting nanoparticles tend to be prohibitively expensive.
By some methods, commercial (bulk) hydrogen sulfide gas is bubbled into a solution containing a metal salt to produce metal sulfide particles. However, the bulk hydrogen sulfide process generally results in a significant proportion of the hydrogen sulfide being wasted to the atmosphere, which is not only costly but presents a significant health risk. Moreover, the bulk hydrogen sulfide process is generally not amenable to producing metal sulfide particles of a specific size. In particular, the bulk hydrogen sulfide process is generally substantially limited in its ability to produce nanoparticles in the lower nanoscale size range, e.g., below 100 nm.
The microbial synthesis of semiconductor nanoparticles is known. These processes are also referred to herein as “conventional nanofermentation” or “conventional NF”. See, for example, P. R. Smith, et al., J. Chem. Soc., Faraday Trans., 94(9), 1235-1241 (1998) and C. T. Dameron, et al., Nature, 338: 596-7, (1989). These processes are generally more capable of controlling the particle size and even shape or crystallinity of the resulting particles. However, there are significant obstacles that continue to hamper such microbially-mediated methods from being commercially viable. For example, current microbial methods are generally limited to the production of semiconductor nanoparticles on a small research scale. In addition, current microbial processes generally produce semiconductor nanoparticles that are contaminated with microbial components (e.g., cell membranes) or components in the culture medium necessary for sustaining the microbes. Accordingly, numerous separation and washing steps are generally required.
Moreover, as the conventional microbial process requires the microbes to be in contact with the metal ions to produce the semiconductor particles, the chemical and physical conditions used in producing the metal semiconductor nanoparticles are substantially limited to those conditions in which the microbes can be sustained. Thus, the conventional microbial process is incapable of optimizing the chemical conditions for growth of the particles in a manner that is independent of the conditions required for sustaining the microbes. For example, the conventional microbial process generally requires a completely aqueous medium, the use of metal ions and metal concentrations well below a lethal limit, and the use of non-toxic chemical components, all of which generally excludes the use of many of the additives and conditions that can be used in purely chemical processes for careful selection of particle size, shape, and crystalline form.